1. Field of the Invention
The present invention relates to an image forming apparatus and scanning method which are applied to a multicolor laser beam printer comprising a plurality of scanners, or the like.
2. Description of Related Art
In earlier technology, various image forming apparatuses for forming an image on an imaging material by modulating a plurality of light beams (laser beams) on the basis of image data, main-scanning the modulated light beams on the imaging material by using a polygon mirror (rotary polygonal mirror), and sub-scanning by rotating the imaging material have been known. Since such image forming apparatuses output a plurality of light beams, they comprise a plurality of scanners. However, registration deviation among each color in each scanner caused by deviation of a write timing signal of sub-scanning caused by phase shift of a polygon mirror driving motor provided in each scanner becomes a problem. In order to obtain multicolor printing of high accuracy by resolving this registration deviation, it is required to correct the registration deviation in sub-scanning direction in an accuracy of not more than one pixel.
Then, as a technique for correcting this kind of registration deviation, in the Japanese Patent Laid-Open Publication No. 7-160084 (hereinafter, it is described as the “foregoing technology 1”), correction data of registration deviation of each scanner to an arbitrary scanner is stored, and the phase of a reference frequency signal of a polygon mirror driving motor is changed on the basis of the correction data. At the same time, the delay time of the count enable signal inputted into a counter section for generating a write timing signal in the sub-scanning direction is switched, and thereby, counting error of the write timing signal in the sub-scanning direction is prevented. Thus, a technique for correcting registration deviation of each scanner is disclosed in the foregoing technology 1.
Further, in the above-mentioned image forming apparatuses, there is a case of performing phase control of a polygon mirror driving motor by using an output signal (FG signal), which is outputted from each hall element corresponding to a rotor of the polygon mirror driving motor or magnetic poles (south pole and north pole) provided in the polygon mirror, as a phase comparison signal of the polygon mirror driving motor. In the above-mentioned foregoing technology 1, the pulse number Pn of the FG signal detected per one rotation of a polygon mirror is subject to being set not more than the number of faces m of the polygon mirror, and the FG signal is used as a phase comparison signal of the polygon mirror driving motor. Thus, a technique of performing phase control of each polygon mirror driving motor in each scanner operated independently is disclosed in the foregoing technology 1.
However, in the image forming apparatus disclosed in the foregoing technology 1, at the time of carrying out initial phase control after a power source is turned on, there is a case that the phase of faces of polygon mirrors cannot be controlled precisely only by fulfilling the above-mentioned relationship of Pn≦m. Further, even though the correction data of registration deviation is stored, for example, in case of performing phase control by restarting the polygon mirror driving motor after rotation of the polygon mirror driving motor in each scanner is stopped temporarily because of jam or the like, also there is a problem that registration deviation cannot be corrected on the basis of the correction data since shift in the phase of faces of the polygon mirror is caused.
The problems in phase control in case of fulfilling the above-described relational expression will be explained as follows. For example, the cases such that the pulse numbers (FG signal) per one rotation are 6 (Pn=6) and 4 (Pn=4) when a polygon mirror has 6 faces (m=6) will be explained as an example. FIG. 7A is a view showing a timing diagram of an index signal (timing signal), FG signal, and clock signal that are outputted during one rotation of a polygon mirror when the polygon mirror fulfills the relationship between Pn=6 and m=6 (Pn≦m), and FIG. 7B is a view showing the same when the polygon mirror fulfills the relationship between Pn=4 and m=6 (Pn≦m).
As shown in FIG. 7A, since the polygon mirror has 6 faces, the reference numbers 1 to 6 in the figure correspond to the number of faces. The index signal is a signal for indicating read timing in one line in the main scanning direction, and corresponds with the number of faces of the polygon mirror by one to one. Further, the FG signal is operated in synchronization with the clock signal which is a reference frequency signal. In this case, when the phase of the clock signal is shifted, the rotation phase of the mirror faces also shifts for that much. Thereby, it can be found that precise phase control of the faces can be carried out. Further, even at the time of restarting, phase control can be realized at the same value on the basis of the previous correction data.
Next, FIG. 7B shows the case that phase control cannot be performed even though the above-described relational expression is fulfilled. The lower stage in FIG. 7B is a view showing a timing diagram in case of shifting the phase of the click signal for φ at the time of initial phase control. As shown in FIG. 7B, when the clock signal is shifted for φ, the phase difference of the following FG signal may become in two ways of φ or 2π−φ. As a result, there exists two kinds of cases, one being the case that the phase difference of the FG signal is φ and the other being the case that the phase difference is 2π−φ, so that there exist two kinds of angles of mirror faces as a result. Therefore, it is impossible to perform the aimed control precisely. That is, it is impossible to perform precise phase control even though the above-described relational expression is fulfilled. Further, in case of restarting the polygon mirror which is made to stop temporarily, since two kinds of angles of mirror faces exists, there is a problem such that aimed phase control of mirror faces cannot be performed.
Moreover, in the image forming apparatuses in the earlier technology, the faces utilized widely in polygon mirrors is 6 faces, and the magnetic poles of the polygon mirrors in this case may be generally 4 poles, 8 poles or 12 poles. When comparing the efficiency of the polygon mirror driving motors with 4 poles, 8 poles and 12 poles, it has a property such that the smaller the number of poles is, the worse the efficiency of torque generation is, and the larger the number of pole is, the larger the switching loss is. Here, deterioration of the efficiency causes rise of driving current value, in particular, when high-speed rotation is required, it accompanies temperature rise of the polygon mirror driving motor. Therefore, a problem is caused in stable control of the polygon mirror. Accordingly, in case of using 6 faces and 8 poles as an ideal relationship between the number of faces and the number of poles of a polygon mirror, there is a problem such that a polygon mirror having 6 faces and 8 poles cannot be used since precise control of phase of faces cannot be performed according to the above-mentioned reasons.